For example, Japanese Unexamined Patent Publication No. 2003-244792 (Patent Document 1) describes a transparent piezoelectric speaker placed in a curved shape on a display of a cellular phone. The piezoelectric speaker includes a flexible piezoelectric film, transparent driving electrodes formed on the opposite surfaces of the piezoelectric film for applying a driving signal voltage thereto, and a protective film overlaid on the piezoelectric film. The Patent Document 1 describes an example where the piezoelectric film included in the piezoelectric speaker is constituted by a PVDF (polyvinylidene fluoride) film.
Organic polymers, such as PVDF described above, have relatively-smaller piezoelectric constants, which are only about 40 pC/N in cases of d33. Therefore, in order to generate practical sound pressures, it is necessary to apply, thereto, voltages of several tens of volts to several hundreds of volts.
On the other hand, such a transparent piezoelectric speaker can attract user's interest due to its transparency and, as a result thereof, it has a greater opportunity of being touched by their hands and, has a higher possibility of being damaged at its surface due to their curiosity. Therefore, the aforementioned protective film may be damaged so that the driving electrode existing therebelow is exposed, which may result in situations which enable the driving voltage to be directly touched. In such cases, electric-shock accidents can be possibly induced, since a relatively-higher voltage is applied to the driving electrodes as described above.    Patent Document 1: Japanese Unexamined Patent Publication No. 2003-244792